1. Technical Field
This invention relates to an optical waveguide for optical amplification, particularly a rare earth element-doped optical waveguide having a core waveguide doped with a rare earth element, and to a process for producing the same.
2. Background Art
Optical fiber amplifiers and fiber lasers in which the core layer of an optical fiber is doped with a rare earth element, such as Er (for amplification at wavelengths around 1.55 .mu.m) and Nd (for amplification at wavelengths around 1.3 .mu.m), are vigorously studied at present for use thereof as optical amplifiers. The optical fiber amplifiers and fiber lasers have the advantages that (1) the core diameter thereof as small as 10 .mu.m ensures an enhanced excitation power density, leading to a higher excitation efficiency, (2) they permit a longer interaction length, (3) they show a very low loss when a silica optical fiber is used therein, and so on.
However, when the optical fiber amplifiers and fiber lasers are mounted together with semiconductor lasers, photodetection devices, optical modulation circuits, optical branching/coupling circuits, optical switching circuits, optical wave mixing/separating circuits or the like to construct a system, there arises the problem that, because of the discrete component parts, it is difficult to obtain a system with a smaller size and a lower loss. In addition, the discrete component parts should be arranged with adjustments of the respective optical axes of the component parts. The adjustments require a very long time, leading to a higher cost, and bring about reliability problems.
Recently, therefore, attention has come to be paid to rare earth element-doped silica optical waveguides (the rare earth element being Er or Nd in most cases) for their potential use as future-type optical amplifiers, in view of the probability that the doped silica optical waveguides can be made smaller and integrated, as contrasted to the optical fiber type amplifiers.
There has been known a process for producing a silica optical waveguide as shown in FIG. 12 (K. Imoto, et al., "Guided-wave multi/demultiplexer with high stopband rejection", Applied Optics Vol. 26, No. 19, October 1987, pp. 4214-4219). The process comprises a series of the following steps (1) to (8):
(1) providing a core glass film 25 (SiO.sub.2 -TiO.sub.2 glass) on a substrate 1 (SiO.sub.2 glass) [FIG. 12(a)], with the refractive-index difference between the glass film 25 and the substrate 1 being about 0.25% and with the thickness T of the glass film 25 being about 8 .mu.m;
(2) heat-treating the thus obtained assembly at a high temperature of about 1200.degree. C. to make the film 25 denser [FIG. 12(b)];
(3) providing a WSi.sub.x film 26 which, to be used for etching the core glass film 25, is about 1 .mu.m thick [FIG. 12(c)];
(4) applying a photoresist to the WSi.sub.x film 26 and patterning the thus formed photoresist film 27 by photolithography [FIG. 12(d)];
(5) patterning the WSi.sub.x film 26 by dry etching, with the patterned photoresist film 27 as a mask [FIG. 12(e)];
(6) patterning the core glass film 25 by dry etching, with the patterned photoresist film 27 and the patterned WSi.sub.x film 26 as a mask [FIG. 12(f)];
(7) removing the photoresist film 27 and the WSi.sub.x film 26 [FIG. 12(g)]; and
(8) providing a clad layer 28 (SiO.sub.2 -P.sub.2 O.sub.5 -B.sub.2 O.sub.3 glass) over the substrate 1 so as to cover the patterned core glass film 25, thereby producing a silica optical waveguide having a substantially rectangular shaped core waveguide 3 in the cladding.
It is difficult for an optical waveguide with a planar structure to be formed into an elongate shape, as in the case of an optical fiber. To obtaining a better excitation efficiency, therefore, an increased amount of a rare earth element should be added to the core of the optical waveguide. It has been found, however, that doping with a large amount of a rare earth element causes concentration extinction, thereby making it impossible to obtain the desired lasing or amplifying function.
The above-mentioned conventional process for producing a silica optical waveguide can be used, with no special problems, where the core glass film 25 is not doped with a rare earth element such as Er and Nd. It has been found, however, that where the core glass film 25 is doped with a rare earth element the conventional process results in roughening of side surfaces of the core waveguide in the step of patterning the core glass film 25 by dry etching, shown in FIG. 12(f). The roughened side surfaces of the core waveguide cause scattering of the propagating light, leading to an energy loss and a lowered optical amplification efficiency. FIG. 13(a) shows an SEM photograph of a rare earth element-doped core waveguide formed by the conventional process, and FIG. 13(b) shows an SEM photograph of a core waveguide not doped with a rare earth element. The photographs show how the side surfaces of the core waveguide doped with a rare earth element is roughened, and also show the deposition of a product, which is considered to be a compound of the rare earth element, on the surfaces of the waveguide.
This is because the rare earth element added to the core glass film 25 is left unetched after the step of patterning the core glass film 25 by dry etching. For instance, when an Er-doped core glass film is dry etched by use of CHF.sub.3 as a reactive gas, Si and Ti are converted into reaction products of high vapor pressures through the reactions: EQU Si+4F*.fwdarw.SiF.sub.4 .uparw. EQU Ti+4F*.fwdarw.TiF.sub.4 .uparw.
and are thereby etched away. On the other hand, Er is converted into a reaction product of a low vapor pressure through the reaction process: EQU Er+3F*.fwdarw.ErF.sub.3
and the reaction product ErF.sub.3 is left unetched. The symbol "*" is used here to indicate that the same discussion applies to the cases where a chlorine-containing etching gas other than CHF.sub.3 is used.